1. Field of the Invention
The present invention concerns a thermal head mounted on a thermal printer for conducting intended printing by heating under the supply of electric current in accordance with printing information.
2. Description of the Prior Art
A thermal head mounted on a thermal printer such as a heat sensitive printer or heat transfer printer has generally been adapted, for example, to arrange a plurality of heat generating elements linearly on an insulative substrate and heat each of the heating elements selectively under current supply in accordance with electric printing information, thereby conducting color developing recording on heat sensitive recording paper referred to as thermal paper in a heat sensitive printer, or melting an ink of an ink ribbon and transferring and recording the same on common paper in a heat transfer printer.
FIG. 16 shows an example of a thermal head of this type in the prior art. A glaze layer 2 made of heat resistant glass which is less heat conductive and functions as a temperature keeping layer is laminated such that the upper surface is flat on the upper surface of an insulative substrate 1 made of alumina or the like, substantially over the entire surface of the insulative substrate 1. A plurality of heat generating elements 3 made of a heat generating resistor material such as Ta.sub.2 N or Ta--SiO.sub.2 are formed linearly on the upper surface of the glaze layer 2 by laminating entirely by means of vapor deposition or sputtering and then etching by photolithography. A common electrode 4a and individual electrodes 4b are formed respectively at the upper surface on both sides of each of the heat generating elements 3 for supplying electric current to each of the heat generating elements 3. Each of the electrodes 4a and 4b is made, for example, of a soft metal such as Al, Cu and Au, laminated entirely to a thickness of about 2 um, for example, by vapor deposition or sputtering and then photolithographically etched and formed into a pattern of a desired shape.
Then, each of the heat generating elements 3 is formed independently and individually such that a heat generating portion 3A corresponding to 1 dot as the minimum printing unit is exposed between the common electrode 4a and individual electrodes 4b. The heat generating portion 3A of each heat generating element 3 is caused to generate heat by applying a voltage between each of the electrodes 4a and 4b.
A protection layer 7 of about 7 to 10 um thickness is laminated on the upper surface of the insulative substrate 1, the glaze layer 2, each of the heat generating elements 3 and each of the electrodes 4a and 4b for protecting each of the heat generating elements 3 and each of the electrodes 4a and 4b. The protection layer 7 comprises an oxidation resistant layer 8 of about 2 um thickness made, for example, of SiO.sub.2 for protecting each heat generating element 3 against oxidative degradation and a wear resistant layer 9 of about 5 to 8 um thickness, for example, made of Ta.sub.2 O.sub.5 laminated on the oxidation resistant layer 8 for protecting each of the heat generating elements 3 and each of the electrodes 4a and 4b against wearing caused by contact with a heat sensitive recording material such as heat sensitive recording paper and an ink ribbon. The protection layer 7 covers all the surface except for terminal portions of each of the electrodes 4a and 4b. The oxidation resistant layer 8 and the wear resistant layer 9 of the protection layer 7 are formed successively, for example, by sputtering or like other means.
However, in the thermal head of the prior art described above, since the thickness for each of the electrodes 4a and 4b are about 2 um, a step is formed between the heat generating portion 3A of each heat generating element 3 and the corresponding top end of the electrodes 4a and 4b. Accordingly, a concave portion formed above the heat generating portion 3A situating between the common electrode 4a and individual electrode 4b can not be eliminated even if the portion thereabove is covered with the protection layer 7. As a result, upon conducting printing by a heat sensitive recording member such as an ink ribbon, there is a problem that heat conduction with respect to the heat sensitive member is poor in the concave portion since it causes a space in which the heat conductivity from the thermal head to the heat sensitive recording member is most reduced and no intact printing can be attained, particularly, to so-called rough paper with roughened surface. Further, since the electrodes 4a and 4b are generally made of a soft material such as Al, the pressing force exerted from a platen as a back-up for the heat sensitive recording member causes deformation, depletion or peeling of the protection layer 7 above the top end of Al or the like constituting the electrodes 4a and 4b, which cause changes in the resistance value of each heat generating element 3 to deteriorate printing or the reduce the reliability and the working life of the thermal head.
In a countermeasure for the problem, as shown in FIG. 17, a protrusion 2A having an arcuate vertical cross section and extending linearly is formed on a substrate 1, a small protrusion 2B of a generally trapezoidal vertical cross section is formed to a thickness of several um at a top portion of the protrusion 2A, and the top ends of the common electrode a and the individual electrode 4b are disposed below the small protrusion 2B for preventing formation of the concave portion in the heat generating portion 3A.
With such a constitution, satisfactory printing quality can be ensured without increasing the application voltage by so much. However, when the small protrusion 2B of the trapezoidal cross section is disposed and the top end for each of the electrodes is disposed at a position lower than the top face of the small protrusion 2B, since steps are present on both sides of the small protrusion 2B, the thickness of the photoresist varies greatly upon forming the heat generating elements 3 by photolithographic etching, and since the accuracy for exposure is lowered along with the occurrence of the gap with respect to the mask, a pattern can not be formed at a high accuracy on the small protrusion 2B to bring about a problem of causing short etching or over etching thereby often varies the resistance value.
In view of the above, for overcoming the foregoing problems in the prior art it has been proposed, as shown in FIG. 18, to form the electrodes 4a and 4b with dual layers in which thin lower individual electrode 5b and a lower common electrode 5a of the lower layer are formed near the top of the glaze layer 2, while main upper individual electrode 6b and upper common electrode 6a of the upper electrode are formed at positions retracting from the top portion thereby reducing the step between the heat generating portion 3A and the electrodes on both sides.
However, in this structure, upon forming the pattern of the lower electrodes 5a and 5b, it is necessary for such etching as not damaging the heat generating element 3 formed therebelow, to bring about a problem that a range for selecting the heat sensitive device material and the electrode material is narrowed. Particularly, if the pattern is intended to be formed by effective dry etching for improving the pattern accuracy, since the Ta--Si--O or Nb--Si--O system as the high specific resistivity material and a low resistance high melting metal such as Mo and W as an effective material for the lower electrode are etched in common by a fluorine series etching gas, it is possible for dry etching by using CF.sub.4 series gas which is simple in view of the process and has been used with much satisfactory result. Further, if the etching for the pattern of the heat generating elements 3 and the etching for the electrode are conducted separately by the prior art method, patterns tend to be deviated in the direction of arranging the heat generating portions 3A between the lower electrode for supplying electric power and the pattern of the heat generating elements 3. Deviation of the pattern, if any, causes short circuit between adjacent heat generation portions 3A or varies the resistance value to lower the yield.
Further, for printing on rough paper at a high speed, it is necessary for so-called hot peeling of peeling and transferring an ink layer of an ink ribbon while the heated ink layer is not yet cooled substantially. Accordingly, it is demanded to shorten the distance from the heat generation portion 3A to the edge of the head on the side of the common electrode as much as possible, but it has been impossible in the prior structure to form the common electrode on the edge at a good accuracy.
Further, in a thermal head of the prior art as shown in FIG. 19, a glaze layer 52 made of a material of low heat conductivity such as heat resistant glass and functioning as a temperature keeping layer is laminated to the upper surface of a substrate 51 made of an insulating material such as alumina, substantially over the entire surface of the substrate 1 such that a portion protrudes upwardly. A plurality of heat generating resistor bodies 53 made of a material, for example, Ta.sub.2 N or Ta--SiO.sub.2 are formed linearly on the upper surface of the protruding portion of the glaze layer 52 by means of vapor deposition, sputtering or the like and then etched into a predetermined pattern by photolithography. Further, individual electrodes 54a connected individually to the heat generating resistors 53, respectively, and a common electrode 54b connected to all of the heat generating resistors 53 are formed respectively to the upper surface on both sides of each of the heat generating resistor bodies 53. Each of the electrodes 54a and 54b is made, for example, of a soft metal such as Al, Cu and Au, laminated by vapor deposition, sputtering or the like and then formed into a desired pattern by photolithographic etching. Then, each of the heat generating resistor bodies 53 is formed independently and individually so as to expose a heat generating portion 53a corresponding to one dot as the minimum printing unit between the common electrode 54 and the individual electrode 54a.
Further, a protection layer 55 is laminated to the upper surface of the substrate 51, the glaze layer 52, each of the heat generating resistor bodies 53 and the each of the electrodes 54a and 54b for protecting each of the heat generating resistors 53 and each of the electrodes 54a and 54b. The protection layer 55 comprises an oxidation resistant layer 56 made, for example, of SiO.sub.2 for protecting the heat generating resistor body 53 from oxidative degradation and a wear resistant layer 57 made, for example, of Ta.sub.2 O.sub.5 laminated on the oxidation resistant layer 56 for protecting each of the heat generating resistors 53 and each of the electrodes 54a and 54b from wearing caused by contact with heat sensitive recording material such as a heat sensitive recording sheet and an ink ribbon. The protection layer 55 covers all the surface other than the terminal portion of each of the electrodes 54a, 54b, and the oxidation resistant layer 56 and the wear resistant layer 57 of the protection film 55 are successively formed by means of sputtering or the like.
In the existent thermal head of this type, electric current is supplied selectively to the individual electrodes 54a based on a predetermined printing information thereby causing the heat generating portion of the corresponding heat generating resistor body 53 to generate heat and conducting color developing recording on a heat sensitive recording sheet or melting the ink of the ink ribbon thereby conducting transfer recording to perform desired printing.
However, in the existent thermal head, since the top ends for the electrodes 54a and 53b are formed on both sides of the protruding portion of the glaze layer 52, steps are formed at the top ends of the electrodes 54a and 54b, and the steps remarkably increase the variation in the thickness of the photoresist upon etching each of the electrodes 54a and 54b by photolithographic etching, and the steps cause a gap relative to the mask to reduce the exposure accuracy, so that the pattern can not be formed at a high accuracy, short etching or over etching is caused to often vary the resistance value.
For dissolving the foregoing problems in the prior art, the present applicant, et al have developed a thermal head having each of the electrodes being formed as dual layers, in which lower individual electrodes and a lower common electrode of the lower layer made of a material, for example, Mo are formed on the lower surface of the heat generating resistor body, and upper individual electrodes and an upper common electrode of the upper layer made of a material, for example, Al are formed on the upper surface of the heat generating resistor body at positions retracting from the top of the protruding portion of the glaze layer, thereby reducing the step between the heat generating portion and the electrodes on both sides.
In such a thermal head, since the step between the heat generating portion of the heat generating resistor body and the electrodes on both sides can be reduced, variation in the thickness of the photoresist can be reduced remarkably and, in addition, the pattern can be formed at a high accuracy, thereby enabling to prevent the occurrence of short etching or over etching to prevent variation of the resistance value.
However, in the thermal head described above, since the heat generating portion of the heat generating resistor body made of a material, for example, Ta--SiO.sub.2 is formed to the upper surface of the glaze layer, and lower electrodes made of a material such as Mo are disposed on both sides of the heat generating portion. Then, since the stresses in the films of the materials for forming the heat generating resistor and the lower electrode are different, there is a problem that adhesion between the lower electrode and the glaze layer is remarkably deteriorated to result in the peeling of the lower electrode and the reduce the head quality.